Method for producing defined polycrystalline silicon areas in an amorphous silicon layer

ABSTRACT

A method of producing well-defined polycrystalline silicon regions is described, in particular for producing electrically conducting regions, having the steps:  
     a) providing a substrate ( 10 ) with an insulating layer ( 12 ) and a layer of doped amorphous silicon ( 14 );  
     b) performing electromagnetic irradiation ( 20 ) using a laser source ( 18 ) to produce the electrically conducting regions ( 26 ),  
     c) positioning a shadow mask ( 22 ) between the laser source ( 18 ) and the substrate ( 10 ) having the layer ( 14 ) for definition of the contours of the electrically conducting regions ( 26 ).

[0001] The present invention relates to a method of producing well-defined polycrystalline silicon areas, in particular for producing electrically conducting regions.

BACKGROUND INFORMATION

[0002] It is known that electrically conducting regions may be defined in an amorphous silicon layer by controlled production of polycrystalline silicon regions. Such polycrystalline silicon regions are characterized by a good electric conductivity, which may optionally be adjusted by introducing suitable dopants. Furthermore, it is known that polycrystalline silicon has a high piezoresistivity, so it is suitable for use of wire strain gauges. Such wire strain gauges are used in pressure sensors, for example. An electric resistance, which may be determined via a corresponding analyzer circuit, changes due to the acting pressure.

[0003] It is known that polycrystalline silicon may be produced by a LPCVD method (low-pressure chemical vapor deposition), where the deposition rate is determined essentially by the process temperature. The process temperatures vary in ranges between 400° C. and 900° C., depending on the layer of polycrystalline silicon to be deposited.

[0004] If such polycrystalline silicon layers are deposited on heat-sensitive substrates, e.g., stainless steel substrates, to produce high-pressure sensors, the high thermal stress associated with such deposition constitutes a high process risk. To define geometrically the electrically conducting regions, it is known that they may be well-defined by photolithographic process steps. This requires that a masking layer be applied to the polysilicon layer and exposed, then the exposed or unexposed regions be removed selectively and next the polysilicon layer must be plasma etched, for example. Such methods are relatively complicated to control and allow only a limited structural fidelity.

ADVANTAGES OF THE INVENTION

[0005] The method according to the present invention having the features characterized in Claim 1 offers the advantage over the related art that a thermal load in production of polycrystalline silicon regions is reduced. Furthermore, an exact structural definition may be achieved so that process reliability and yield are increased. In situ high resolution structuring of the polycrystalline silicon regions in the submicrometer range is possible due to the fact that a substrate is provided with a layer of a doped amorphous silicon, the amorphous silicon is irradiated using a laser source to produce the electrically conducting regions, a shadow mask being positioned between the substrate and the laser source to provide definition of the electrically conducting regions. Irradiation of the doped amorphous silicon using a laser source, in particular an excimer laser, permits a controlled breakup of the bond structure of the amorphous silicon through direct electronic energization and production of a polycrystalline lattice structure as a function of the wavelength used and the duration of the laser treatment. Polycrystalline silicon having a high electric conductivity, a low temperature dependence of the resistance, and a marked piezoresistivity is obtained by previously doping the amorphous silicon. Use of the shadow masks in laser treatment also offers the advantage that photolithographic process steps are not necessary. This reduces manufacturing costs on the whole.

[0006] Preferred embodiments of the present invention are derived from the other features characterized in the subclaims.

DRAWING

[0007] The present invention is explained in greater detail below in exemplary embodiments on the basis of the drawing.

[0008]FIGS. 1a through 1 e show schematic manufacturing steps in the production of polycrystalline silicon regions in a first variant, and

[0009]FIGS. 2a through 2 f show schematic process steps for production of polycrystalline silicon regions in a second exemplary embodiment.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0010]FIGS. 1a through 1 e show schematically individual process steps in the production of well-defined polycrystalline silicon regions by the method according to the present invention. A silicon oxide (SiO₂) layer 12 is first applied to a substrate 10, e.g., a stainless steel substrate. Then as illustrated in FIG. 1b, a layer 14 of doped amorphous silicon is deposited on this layer 12. Then as illustrated in FIG. 1c, a passivation layer 16, e.g., of silicon dioxide (SiO₂) or silicon nitride (Si₃N₄) is applied.

[0011] In a next step illustrated in FIG. 1d, the composite of layers 10, 12, 14, 16 is irradiated with a laser source 18, e.g., an excimer laser, using electromagnetic radiation 20. A shadow mask 22 having at least one mask opening 24 is situated between laser source 18 and the composite of layers 10, 12, 14, 16. In the area of mask opening 24, electromagnetic radiation 20 strikes the composite of layers 10, 12, 14, 16. Passivation layer 16 is transparent to electromagnetic radiation 20. Crystallization occurs in the area of amorphous silicon layer 14—well-defined geometrically by the mask opening—due to irradiation with electromagnetic radiation 20, so that a polycrystalline silicon region 26 develops there. According to the contour of mask opening 24, region 26 of the polycrystalline silicon is defined and is embedded in layer 14 of amorphous silicon. Due to the doping of amorphous silicon 14, an electrically highly conductive polycrystalline silicon region 26 is formed. Since amorphous silicon 14 has a relatively high resistance and polycrystalline region 26 has a high electric conductivity, the electrically conductive regions are well-defined by region 26.

[0012] Then as illustrated in FIG. 1e, contact windows 28 are opened in passivation layer 16, and a metallic coating (not shown here) is subsequently deposited in these windows. This metallic coating provides contacting of electrically conducting region 26.

[0013] Contact windows 28 may likewise be opened by irradiation with a laser light. It is possible in this way to selectively open contact windows 28 by changing the wavelength of the laser light, for example, and/or increasing the power of laser source 18 and providing a suitable shadow mask.

[0014] It is apparent on the basis of the schematic diagram that photolithographic process steps are not necessary for production of well-defined electrically conductive regions 26 of polycrystalline silicon. Furthermore, irradiation with laser light is not critical thermally, so that substrate 10 is not exposed to an excessive thermal load. In this way, electrically conducting regions 26 may be produced with a high process reliability and a high process rate.

[0015] The method according to the present invention may be used, for example, in the production of high-pressure sensors in which substrate 10 is made of a stainless steel and electrically conducting regions 26 form wire strain gauges in a bridge circuit (e.g., a Wheatstone bridge). This requires only an appropriately adapted design of shadow mask 22, which has an appropriate number of mask openings 4 (e.g., four in this case) for definition of the bridge resistors and corresponding openings to form the feeder lines (printed conductors). In the case of high-pressure sensors having stainless steel substrates or other structural components having non-silicon wafer substrates, the method according to the present invention is especially advantageous because it eliminates the use of conventional photolithography which with these components is a yield-limiting process that is difficult to control.

[0016]FIGS. 2a through 2 f illustrate in a modified variant the method according to the present invention. The same parts as in FIG. 1 are provided here with the same reference numbers and will not be explained again here, so that only the differences will be discussed.

[0017] In contrast with the embodiment illustrated in FIGS. 1a through 1 e, polycrystalline silicon region 26 is structured before deposition of passivation layer 16. This makes it possible, as illustrated in FIG. 2d, to selectively remove the regions of amorphous silicon (former layer 14) surrounding polycrystalline silicon regions 26 produced then. Because of the prevailing etching selectivity between amorphous silicon and polycrystalline silicon, which is particularly pronounced in the case of strong boron doping, this may be implemented by an etching attack, e.g., through the use of plasmas containing hydrogen or halogen, in a simple manner without a photolithography step. Following this, as illustrated in FIG. 2e, passivation layer 16 is deposited and then (FIG. 2f) contact windows 28 are structured therein. These windows are then metal plated again so that polycrystalline silicon regions 26 may be connected to an electric circuit.

[0018] It is clear on the whole that, in the first embodiment, the process steps illustrated in FIGS. 1a, 1 b and 1 c and the embodiments illustrated in FIGS. 2a and 2 b may be performed immediately in succession in one recipient without any negative effect on the vacuum atmosphere required in the meantime, or at least having to release the vacuum. This yields on the whole a shorter process running time. An essential advantage is also in particular the fact that a thermal stress on substrate 10 is greatly reduced in comparison with the known LPCVD deposition method for polycrystalline silicon. In addition, due to the prior doping of amorphous silicon in layer 14 and the subsequent well-defined exposure of regions 26 to electromagnetic radiation 20, very homogeneous polycrystalline silicon regions 26 are obtained, resulting in a considerable reduction in asymmetry in the entire bridge when used as wire strain gauges in a Wheatstone resistance bridge, so that high-precision piezoresistive pressure sensors may be produced through this method according to the present invention. 

What is claimed is:
 1. A method of producing well-defined polycrystalline silicon regions, in particular for producing electrically conducting regions, having the steps: a) providing a substrate (10) with an insulating layer (12) and a layer of doped amorphous silicon (14); b) performing electromagnetic irradiation (20) using a laser source (18) to produce the electrically conducting regions (26), c) positioning a shadow mask (22) between the laser source (18) and the substrate (10) having the layer (14) for definition of the contours of the electrically conducting regions (26).
 2. The method according to claim 1, wherein the electromagnetic irradiation (20) is performed with an excimer laser.
 3. The method according to one of the preceding claims, wherein the layer (14) of doped amorphous silicon is provided with a passivation layer (16) before the electromagnetic irradiation (20).
 4. The method according to claim 3, wherein silicon dioxide (SiO₂) or silicon nitride (Si₃N₄) is applied as passivation layer (16).
 5. The method according to one of claims 1 or 2, wherein before applying a passivation layer (16), the remaining amorphous regions (14) are removed selectively in a plasma containing hydrogen or halogen without using a photolithographic step.
 6. The method according to one of the preceding claims, wherein contact windows (28) are created in the passivation layer (16) for electric contacting of the regions (26).
 7. The method according to claim 6, wherein the contact windows (28) are created through electromagnetic radiation by using a laser source.
 8. The method according to one of the preceding claims, wherein the regions (26) and the contact windows (28) are produced by using the same laser source (18) but different operating parameters, in particular different wavelengths and/or irradiation times and/or irradiation intensities.
 9. A high-pressure sensor having at least one wire strain gauge made of polycrystalline silicon obtainable according to at least one of claims 1 through
 8. 